. Shield and compressed air tunneling . om will be found. 29. Observation Towers.—Much ingenuity often has to beexercised in the choice of the base line terminals so that the otherobservation points are visible from them. Sometimes the ter-minal points are transferred to observation towers, that is to say,the terminal point having ])een marked out on the ground with 37G SHIELD TUNNELING permanent monuments, a timber or steel frame tower is erectedover it. These towers are made double, one tower withinanother. The outer tower supports the observer, the inner theinstrument. The construction must
. Shield and compressed air tunneling . om will be found. 29. Observation Towers.—Much ingenuity often has to beexercised in the choice of the base line terminals so that the otherobservation points are visible from them. Sometimes the ter-minal points are transferred to observation towers, that is to say,the terminal point having ])een marked out on the ground with 37G SHIELD TUNNELING permanent monuments, a timber or steel frame tower is erectedover it. These towers are made double, one tower withinanother. The outer tower supports the observer, the inner theinstrument. The construction must be most rigid to obviatevibrations, which will occur to some degree in any case. Theposition of the monument is transferred to the table which sup-ports the instrument by means of two series of transit observa-tions made at right angles to each other. 30. Stations on Roofs of Buildings.—Sometimes in built uptowns even such towers are not possible and the triangulationstations have to be placed on the roofs of buildings. This makes. hpofBldg. \\sS^^>S\\\\% f Fig. 147.—Method used when the triangulation stations are on the roofs of buildings. it impossible to measure directly the distance between the ter-minal points. This has to be obtained by a secondary triangula-tion, based on a measured distance made on the ground asconveniently as possible to the two points on the buildings, as shown in Fig. 147. The distances\-A and 3-5 are measured. This may have to be done intwo sections o and h as shown inFig. 148. In this way a quad-rilateral is set up, \-Z-B-A in whichthree sides and four angles maybe observed and consequently thefourth side 1-3 which is the baseline may be computed. The inter-section of the tunnel center line aslaid out on the ground with thesecondary base line is monumentedas shown at C on Fig. 147, thedistances A-C and B-C measured and the angle at C ties in the stationing of the tunnel center line with thetriangulation. , Sireef Fig. 148.
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